When disease or trauma causes a skeletal void, or whenever healing of a fracture is impaired, a common surgical technique involves harvesting of bone from the iliac crest and transferring the bone graft to the injury site. This is referred to as a bone autograft. The autograft is used to fill the bone void providing osteoconductive and osteogenic properties necessary for effective healing of the injury. This technology is generally regarded as the golden standard in orthopedic surgery but carries severe drawbacks due to frequent morbidity associated with the bone harvest. Hence, bone graft substitutes, possessing osteoconductive and osteogenic properties have been desired by industry and academia alike aiming to eliminate the need for bone autografts. Such bone graft substitutes are generally referred to as synthetic bone graft substitutes. So far, no single product has appeared, proven to provide osteoconductive as well as osteogenic properties to a substantially bone deficient injury side.
Chitin is a natural biopolymer which is obtainable from crustacean shell, but can also be obtained from other invertebrates and from fungi. By deacetylation of the N-acetyl glucosamine residues of the chitin polymer, typically by hydrolyzing the N-acetyl linkages with concentrated alkali, chitosan is obtained. By definition, chitosan is generally described as a copolymer of D-glucosamine (D) and N-acetyl-D-glucosamine (A), which is insoluble in water at pH above 6.2—the isoelectric point of the free amine group—but dissolves at pH below about 6.2. Typically, about 75-100% of the monomeric units in conventional chitosan copolymer are D-glucosamine, which can be described as 75-100% deacetylated chitosan, or as having a degree of deacetylation (DD) of 75-100%. Accordingly, 0-25% of the monomers in such material are N-acetyl-D-glucosamine groups (A).
When the degree of deacetylation is lower than about 75%, the chitin polymer displays different solubility properties, such material, with DD from about 75% down to about 40%, is generally referred to as partially deacetylated chitin, referred to herein as PDC.
The present inventors have previously described biological properties of partially deacetylated chitin polymer and oligomers. WO 03/026677 describes the use of PDC oligomers for treating rheumatoid conditions. WO 2006/134614 discusses biological properties of partially deacetylated chitin polymer and oligomer and discusses how such oligomers act as blockers for chitinase enzymes.
Chae Cho et al. (J. Craniofacial Surgery Vol. 16 No 2 Mar. 2005) describe experiments with solid pellets with chitosan-calcium sulfate composite and their effects on the osteogenesis of defective tibia in rabbits, using chitosan with 90% DD.
Yamada et al. (J. Biomed. Mat Research Vol. 66A no. 3, 1 Sep. 2003, pp 500-506) investigate and discuss effects of chitosan on biologic mineralization and investigated effects of chitosan supplemented into culture medium on osteoblasts.
Klokkevold et al. (J Periodontol. 1996 November; 67(11):1170-75) evaluated the effect of chitosan on the differentiation of osteoprogenitor cells.
WO 2004/028578 discloses a composition for stimulating bone-formation and bone-consolidation that comprises bone morphogenic protein (BMP) as an active ingredient in a formulation with solutions of tripolyphosphate and chitosan, where the two solutions will instantly solidify upon mixing, causing specific handling concerns in the clinic.
WO 01/41822 describes self-gelling mineral-polymer hybrid formulations that comprise (i) a water-based thermogelling liquid component that comprises dissolved chitosan and has a pH between 6.5 and 7.4, and (ii) a solid component comprising calcium, said liquid and solid components are to be mixed together to form a non-hardening thermo-gelling composition, which forms a gel at body temperature.
WO 2006/057011 discloses solid implants of co-precipitated hydroxyapatite and chitosan in 1:1 ratio (chitosan having a degree of deacetylation of 87%), and compositions with “putty”-like consistency, containing a total of 5% chitosan, calcium phosphate and polydimethylsiloxane, and a solid-to-liquid ratio of 2:1. Setting time of the putty-paste was a little less than 6 minutes. The paste was tested for healing induced bone injury in rats.
It is generally held in the art that chitosan itself is not osteoconductive (see e.g. Venkatesan and Kim, “Chitosan Composites for Bone Tissue Engineering—An Overview”, Mar Drugs 2010; 8(8): 2252-2266.
Further development of alternative and more effective products, practical for clinical applications is still much appreciated, for effective and low cost bone healing treatments.